If you have a good moveset for Volcanion, post an answer below and upvote the best ones. Remember, this is for competitive movesets, not in-game. Ability, EVs etc should be included. Make sure to read all the guidelines here.

I've found that if you use Sludge Bomb, you will have no Pokemon that resist all four of your moves! The type coverage that Water, Fire, Ground, and Poison bring give 354 Pokemon normal damage, and will hit 451 Pokemon super effectively! Flash Cannon is an option as well, but you'll get 7 Pokemon that can resist you (such as Gyarados)

Stallbreaker Volc is surprisingly effective. It has more longevity than outright wallbreaker variants whilst shutting down stallmons without too much effort. Overheat is a super powerful move that punches through teams and deters any sort of switch in. Haze stops Calm Minders setting in your face, but also negates the Special Attack drop from Overheat, which is a nice benefit. The next two slots are down to preference. Toxic limits Quagsire and generally crippes defensive mons. Will-O-Wisp does the same but slower whilst screwing up offensive switch ins. Scald is a nice secondary attack to have with a good burn chance. HP Grass is for Quagsire primarily. Fourth slot is the same, but Steam Eruption is a fairly good move choice here to act as backup, no penalty firepower. EVs and nature make it hard hitting enough versus defensive mons while still being able to sponge off neutral and weak hits without effort.

This set will tear through teams. Scald > Steam Eruption for more BP and the same chance to burn. Flamethrower / Fire Blast is the normal STAB Fire attack, with Flamethrower being a higher PP, longer lasting move while Fire Blast is guns-blazing destroy everything move. Earth Power is for coverage, and Overheat or Sludge Bomb is the last move. SB is used for dealing with Faires, and Overheat allows Volcanion to have a nuke option, basically spewing out a 195 powered fire move directly into the opposition's face.

Moveset

Power Herb Volcanion actually works a decent lure for bulky waters, allowing it to kill with Solar Beam. This can be pretty useful if you find your team weak to such threats. The rest are pretty self explanatory. Fire Blast and Steam Eruption are Dual Stabs, while the last two slots are dedicated to coverage moves of your choice. The EVs are so that Volcanion does not take exactly 25% from SR damage and maximizes its Sp. Atk.

Other Options

You could also run 4 Def / 252 Sp. Atk / 252 Speed , but Volcanion's speed is rather pitiful. Another possibility would be to run Rash Nature with Superpower to get Chansey switch-ins, but it still only does around 30 - 40 % without any Attack investment.

Specs Volcanion is also a great set because it becomes a great wall-breaker.

Air Balloon Volcanion is also another possibility, as it offers a switch-in to Ground types and possibly KO them with Steam Eruption.

Steam Eruption & Overheat are good STABs, earthquake covers electric and rock, and Mist keeps special attack same after Overheat. I pair this with another Pokemon that has a move that lowers its own stats.